This study was designed to investigate the effects of exercise training and detraining (cessation of exercise) on the cardiac performance of the male rat heart and to correlate performance changes with structural changes. Th exercise regimen used produces significant increases in heart weight-to-body weight ratios, and a 50% increase in the succinic dehydrogenase activity of the gastrocnemius muscle of the rats run on a treadmill, indicating that the training program is sufficiently strenuous to produce a training effect. Despite the hypertrophy produced by the exercise, pump performance of the exercise-trained hearts and their muscle mechanics (evaluated after 12 weeks of training, using the isolated perfused heart preparation and the papillary muscle preparation, respectively) were not different from control sedentary hearts. RNA, DNA, and hydroxyproline concentrations were also similar to controls. Studies of protein turnover during the early phase of adaptation to exercise are currently in progress.